Transformers

Abstract

A multi-transformer unit is operative for converting an input AC voltage at a first voltage level to a net output AC voltage at a second voltage level. The multi-transformer unit can be used in place of a conventional transformer or an AC machine and is designed to suppress the transfer of harmonics between the input and output AC voltages. The unit comprises at least two phase-shifting transformers, each transformer being operative to provide a phase shift relative to the first voltage level. On the primary side of the unit the transformers are arranged for independent connection to the first voltage level, whereas on the secondary side of the unit the transformers are linked such that voltage vectors on the secondary side of the unit are added together to at least partially cancel the harmonic pollution and give the net output AC voltage. Moreover, the phase shift of the phase-shifting transformers may be selected to completely or substantially add into the net AC output voltage of the multi-transformer unit the fundamental voltage/frequency applied to its input AC voltage.

1 - 15 . (canceled) 
     
     
         16 : A multi-transformer unit ( 20 ) for converting an input AC voltage at a first voltage level to a net output AC voltage at a second voltage level, the input voltage being polluted with at least one harmonic, the unit having primary and secondary sides and comprising at least two phase-shifting transformers, each transformer being operative to provide a phase shift relative to the first voltage level, wherein on the primary side of the unit the transformers are arranged for independent connection to the first voltage level and on the secondary side of the unit the transformers are linked such that voltage vectors on the secondary side of the unit are added together to at least partially cancel the harmonic pollution and give the net output AC voltage. 
     
     
         17 : The multi-transformer unit according to  claim 16 , in which the phase-shifts of the transformers are selected to add into the net output AC voltage a significant voltage at the fundamental frequency of the input AC voltage. 
     
     
         18 : The multi-transformer unit according to  claim 16 , in which the phase shifts of at least a first and second of the phase-shifting transformers are different. 
     
     
         19 : The multi-transformer unit according to  claim 16 , in which at least first and second phase-shifting transformers have primary and secondary windings and the secondary windings of at least the first and second phase-shifting transformers are connected together in series to give the net output AC voltage. 
     
     
         20 : The multi-transformer unit according to  claim 19 , in which at least one of the primary and secondary windings of at least one of the phase-shifting transformers includes a set of main windings and a set of extended windings. 
     
     
         21 : The multi-transformer unit according to  claim 20 , in which a set of extended windings of a first phase-shifting transformer and a set of extended windings of a second phase-shifting transformer have a different number of turns such that the phase shifts they contribute to the net output AC voltage of the unit are different. 
     
     
         22 : The multi-transformer unit according to  claim 19 , in which connections of the primary and secondary windings of the phase-shifting transformers are selected from the group consisting of:
 the primary windings connected together in a star configuration and the secondary windings connected together in an extended star configuration;   the primary windings connected together in an extended star configuration and the secondary windings connected together in a star configuration;   the primary windings connected together in an extended star configuration and the secondary windings connected together in a extended star configuration;   the primary windings connected together in a delta configuration and the secondary windings connected together in a star configuration;   the primary windings connected together in a delta configuration and the secondary windings connected together in an extended star configuration;   the primary windings connected together in an extended delta configuration and the secondary windings connected together in a star configuration; and   the primary windings connected together in an extended delta configuration and the secondary windings connected together in an extended star configuration.   
     
     
         23 : The multi-transformer unit according to  claim 19 , in which at least one of the phase-shifting transformers also includes a set of tertiary windings. 
     
     
         24 : The multi-transformer unit according to  claim 16 , consisting of two phase-shifting transformers. 
     
     
         25 : The multi-transformer unit according to  claim 16 , in which the phase-shifting transformers have a common magnetic core. 
     
     
         26 : The multi-transformer unit according to  claim 16 , arranged to give a net output AC voltage of lower value than the input AC voltage. 
     
     
         27 : The multi-transformer unit according to  claim 16 , arranged to give a net output AC voltage at least equal in value to the input AC voltage. 
     
     
         28 : A method of converting an input AC voltage at a first voltage level to a net output AC voltage at a second voltage level by means of a multi-transformer unit, the input voltage being polluted with at least one harmonic, the unit having primary and secondary sides and comprising at least two phase-shifting transformers, the method comprising the steps of pre-selecting a phase shift relative to the first voltage level for each phase-shifting transformer, operating each phase-shifting transformer to provide the pre-selected phase shift on its secondary side, and adding voltage vectors from each phase-shifting transformer on the secondary side of the unit to at least partially cancel the harmonic pollution and give the net output AC voltage. 
     
     
         29 : The method according to  claim 28 , in which the step of pre-selecting the phase shifts of the phase-shifting transformers includes the step of selecting the phase-shifts to add into the net output AC voltage a significant voltage at the fundamental frequency of the input AC voltage. 
     
     
         30 : The method according to  claim 28 , in which the step of pre-selecting the phase shifts of the phase-shifting transformers includes the steps of:
 determining a Difference Angle according to the formula, Difference Angle=180 degrees/N, where N is a given harmonic of the fundamental frequency of the input AC voltage; and   selecting the phase shifts of the first and second phase-shifting transformers such that the angle between them is substantially equal to the Difference Angle.